The prior art includes a large amount of literature about the use of metal-containing zeolite materials as catalysts or as adsorbents. For example, metal-doped zeolite materials are used as catalysts in the selective catalytic reduction (SCR) of nitrogen oxides to nitrogen and water in emission control technology.
For example, U.S. Pat. No. 4,961,917 describes the use of iron- or copper-doped zeolites in a catalytic process for reduction of nitrogen oxides in the presence of ammonia and oxygen. The catalyst described is a zeolite with a silicon dioxide to aluminum oxide ratio of at least 10. This zeolite has a pore structure which is bonded in all three crystallographic dimensions by pores, which have an average kinetic pore diameter of at least 7 Å. The iron and/or copper promoters are present in an amount of 0.1 to 30% by weight of the total weight of promoter plus zeolite. The zeolite is selected from the group consisting of USY, beta and ZSM-20. The iron or copper sources used are sulfates.
Owing to the harmful effect of nitrogen oxide emissions on the environment, it is an important concern to further reduce these emissions. For the near future, significantly lower NOx limits for exhaust gases stationary systems and motor vehicles from than the present standard are already envisaged.
The removal of nitrogen from combustion gases is also referred to as DeNOx. In auto technology, selective catalytic reduction (SCR) is one of the most important DeNOx techniques. The reducing agents used are typically hydrocarbons (HCSCR) or ammonia (NH3—SCR), or NH3 precursors such as urea (Ad-Blue®). In this context, metal-doped zeolites have been found to be very active SCR catalysts which are usable within a wide temperature range.
Customary processes for doping zeolites with metals include, for example, methods such as liquid ion exchange, solid phase ion exchange, vapor phase ion exchange, mechanical-chemical processes, impregnation processes, and the so-called ex-framework processes.
At present, the doping is undertaken predominantly via liquid ion exchange. First, the zeolite material is prepared in a hydrothermal synthesis, crystallized and calcined. The calcination burns off the organic constituents, and the zeolite material is typically obtained in the H or Na form. After the calcination, ammonium ions are exchanged into the zeolite material, the zeolite is calcined again and then the desired metal ions are exchanged in.
Also known is the doping of zeolites with iron by solid-state ion exchange (EP 0 955 080 B1), wherein a mixture of the desired zeolite, a metal compound and an ammonium compound is sintered under a protective gas atmosphere, such that metal-containing catalysts with an increased long-term stability are obtained.
Problems arise especially in the case of doping or introduction of the doping metals into the zeolite, since different oxidation states of these catalytically active metals are often present alongside one another and the desired catalytically active species is not always obtained, or the catalytically active species are converted to catalytically inactive species owing to the reaction conditions of the doping process.
However, it has been found that virtually all known prior art processes form cluster species of the catalytically active metals by metal exchange in the interior of the zeolite, said cluster species being catalytically inactive or lowering the catalytic activity to an extreme degree. In addition, the clusters have an adverse effect on the stability of the zeolite material. The term “cluster” is understood to mean polynuclear bridged or unbridged metal compounds which comprise at least three identical or different metal atoms.
Inactive metal clusters, moreover, lower the pore volume and hinder gas diffusion, or lead to undesired side reactions.
WO 2008/141823 for the first time discloses metal-containing zeolites in which no metal clusters are detectable in the interior of the zeolite framework. It is stated that the metal-exchanged zeolite is free of catalytically inactive or catalytically less active metal clusters, such that only monomeric or dimeric, highly catalytically active metal species are present in the pore structure. These zeolites can be obtained by first preparing an aqueous or water-comprising slurry of a zeolite and then a) increasing the pH of the slurry to a value in the range from 8 to 10, preferably using NH4OH and with adjustment of the oxygen content in the reaction vessel to a value of <10%, b) lowering the pH to a value in the range from 1.5 to 6, c) adding a metal salt and converting over a period of 1 to 15 hours, d) filtering off and washing the metal-doped zeolite.
A further problem with the aqueous ion exchange is that the metal concentration at the surface is typically higher than in the interior of the zeolite material. Consequently, the aqueous ion exchange leads to an inhomogeneous distribution of the dopant metals in the zeolite material.
A disadvantage of the zeolite doping processes described is, however, that the particular maximum amount of doping metals to be absorbed is limited by the number of cationic positions of the particular zeolites. It follows from this that, for an application which requires a particular amount of dopant metal, not all zeolites are available, but only those which have the desired number of cationic positions. A further disadvantage is that the zeolites which have a higher number of cationic positions and can accordingly absorb a greater amount of dopant metals are less stable (for example after aging) than those with a lower number of cationic positions.
A further disadvantage of the zeolite doping processes described is that these doping processes have many reaction stages, and each reaction stage can damage the zeolite framework and consequently reduce the specific surface area and hence the hydrothermal stability.
It has remained unappreciated to date in the prior art of DeNOx SCR technology that iron pentacarbonyl is suitable as an iron source in the preparation of iron-doped zeolites.
U.S. Pat. No. 2,533,071 already describes the preparation of metallic iron catalysts by heating of iron pentacarbonyl on a support, such that iron pentacarbonyl decomposes to iron and CO and iron is deposited on the support. The catalyst is used to synthesize hydrocarbons from CO and H2. Synthetic spinels are described as preferred support. Additionally mentioned are compositions composed of, for example, 12.5% silicon oxide and 87.5% aluminum oxide.
In addition, U.S. Pat. No. 4,003,850 describes a process for preparing iron oxide catalysts, wherein a suitable support absorbs iron pentacarbonyl and then the iron pentacarbonyl is oxidized to iron oxide. The support described includes zeolites. The use for reduction of nitrogen oxides from exhaust gases with the aid of carbon monoxide at a pressure of greater than or equal to 1 bar is described. In the examples of U.S. Pat. No. 4,003,850, Alcoa H-151 (activated aluminum oxide), Harshaw AL-1602 (silicon aluminum oxide with 91 Al2O3, 6 SiO2), Alcoa F-1 4-10 (activated aluminum oxide), Linde 13X (zeolite with Na2O.Al2O3.2.5SiO2) and Hatshaw Fe-0301(iron-containing activated aluminum oxide) are used.
CN 101099932 A describes the preparation of iron-doped catalysts, wherein the iron particles have a particle size of less than 100 nm. The catalysts are prepared using iron pentacarbonyl, which decomposes in situ to iron. Uses specified for these iron-doped catalysts are chemical processes for coal conversion (e.g. coal liquefaction), petroleum refining and ammonia synthesis. The process for preparing these iron-doped catalysts comprises several stages: (i) transferring the catalyst support into an autoclave, placing it under reduced pressure or replacing the air in the autoclave with nitrogen or inert gas; (ii) adding iron pentacarbonyl; (iii) heating up to a temperature and holding at this temperature, at which iron pentacarbonyl evaporates and penetrates into the catalyst support; (iv) further heating or introducing nitrogen or another inert gas by means of high pressure, such that the iron pentacarbonyl present in the support decomposes in situ to iron which has particle sizes in the nanometer range. Possible supports include zeolites, activated carbons, γ-Al2O3, kieselguhr and carbon.
WO 98/57743 describes the use of iron-doped zeolites, which have been prepared, inter alia, using iron carbonyls as the iron source, as a catalyst in the conversion of synthesis gas to olefins, especially ethylene, propylene and butene. In the examples, ZSM-5, SAPO-34 and SAPO-17 are used.
In spite of extensive literature in the field of doping of supports via gas phase reaction, no use of this process for the preparation of SCR catalysts has been described to date. Moreover, the potential of the gas phase reaction with regard to a loading with dopant metals going beyond the loading limited by the cationic positions is yet to be discovered.